As far as I know, the /5 and /2 HL buckets are interchangeable, except the /5 has a cool little tach inside the speedometer. Those assemblies are worth your left leg, BTW, because a lot of people putting newer engines in the /2 frames like having a tach while keeping the proper look. AS you said, the "key" (which fits every /5 and /2 ever made) just pushes down or pulls up on the top, in front of the speedometer.
One of the best BMWs ever made, IMO.
I don't know what a Viper looks like, but Givi makes a cool little flyscreen.